Automated systems for measuring the concentration of analytes in a sample have been developed using a number of analytical techniques such as chromatography or mass spectrometry. In particular, mass spectrometry is often the technique of choice to achieve sensitivity of parts per billion (ppb) or sub-ppb such as parts per trillion (ppt). For example, co-assigned U.S. patent application Ser. No. 10/086,025 (the '025 application) discloses an automated analytical apparatus measuring contaminants or constituents in trace concentrations.
In an Isotope Dilution Mass Spectrometry (IDMS) technique, a sample of interest is spiked, i.e., has added to it a known amount of the appropriate isotopic species. In measuring trace concentrations, the spike source will generally be stored at a relatively high concentration and must then be diluted before use. Accordingly, the '025 application discloses a dilution module tat includes a reservoir of spike solution stored at a stable, relatively high concentration. A syringe pump is used to remove a portion of spike from the reservoir, which is then mixed with a diluent sample in a mixer. Because the automated apparatus disclosed in the '025 application was directed to the measurement of constituents or contaminants at trace concentrations, there was no need to dilute the sample before mixing it with the spike.
However, there are applications in which dilution of the sample is necessary. For example, copper processing in semiconductor manufacturing uses a relatively comprises a relatively concentrated acidic aqueous copper sulfate solution. Plating topology is controlled by organic plating solution additives within the copper sulfate solution that function to either suppress or accelerate the plating process. These additives experience electrochemical breakdown during the plating process and can be lost by drag out or by becoming trapped within the film. However, the achievement of void-free plating in the vias and trenches of sub-micron high-aspect-ratio structures requires very tight control of additive levels. Unlike indirect measurement methods such as cyclic voltametric stripping (CVS) that monitor the effectiveness of the plating solution, the IPMS apparatus discussed above allows a user to directly measure the additive concentration plus the breakdown products in the electroplating bath to ensure a defect-free deposition process.
Since the electroplating process takes place under clean room conditions, automation to minimize human interaction with the metrology tool is critical. The in-process mass-spectrometry (IPMS) apparatus disclosed in the '025 application meets this automation need but does not provide a capability to dilute the sample and spike simultaneously. Moreover, the dilution module disclosed in the '025 application uses a syringe pump to draw a portion of the spike prior to its dilution. Because of mechanical vagaries, a syringe pump will not necessarily draw the same amount for each portion, thereby adversely affecting measurement precision. In contrast, loop dilution techniques avoid this imprecision through the use of two-position multi-way valves.
A conventional two-position eight-way valve 10 is shown in FIGS. 1a and 1b. Each way or port of valve body 11 is numbered, starting from port 1 through port 8. A loop or fluid conduit 13 keeps ports 7 and 4 connected (in fluid communication). Depending upon whether valve 10 is in a load and delivery position as seen in FIG. 1a or in a mix position as seen in FIG. 1b, similar fluid connections between other ports may be changed. For example, in the loading position (FIG. 1a), ports 6 and 7 are in fluid communication whereas in the mix position (FIG. 1b) ports 6 and 7 are in fluid communication with ports 5 and 8, respectively. Port 1 is closed in both phases. During the loading phase shown in FIG. 1a, loop 13 is filled with the solution-to-be-diluted by pumping into port 6 from a solution source (not shown) connected to line 14 which in turn is connected to port 6. To ensure a clean sample within loop 13, this pumping continues for a sufficient amount of time to flush any previous solution within loop 13 out through port 4 into port 5 which in turn is connected to an output line 15. Note the advantages of such a loading phase: the internal volume of loop 13 is static and thus the volume of solution loaded into loop 13 will be constant for each loading stage or cycle. This fixed volume of solution stored within loop 13 will then be diluted in the loading stage shown in FIG. 1b. In this loading stage, one end of loop 13 is now connected to syringe pump 12 through port 3. The remaining end of loop 13 connects to a conduit 17 connected to a diluent source (not illustrated) through port 8. Thus, as the plunger in syringe pump 12 is withdrawn, turbulent mixing of the solution which had filled loop 13 with the diluent drawn through port 8 occurs within syringe pump 12. This mixing is aided by a reciprocating movement of the plunger. Because syringe pump 12 may be controlled by a stepper motor, the volume of the fluid withdrawn into syringe pump 12 may be fairly precisely reproduced during subsequent mixing phases. Finally, valve 10 returns to the loading and delivery position of FIG. 1a so that the plunger of syringe pump 12 may be depressed, thereby pumping the diluted solution out through port 2.
Although the loop dilution technique described with respect to FIGS. 1a and 1b are efficient and reasonably precise, conventional loop dilution valves do not allow a user to simultaneously mix and dilute two different solutions (such as a sample and a spike in an IPMS process). Accordingly, there is a need in the art for improved loop dilution valves and techniques permitting the precise mixing and simultaneous dilution of two different solutions.
Such an improved loop dilution valve may be used in an in-process mass spectrometry (IPMS) system to mix and dilute sample and spike before analysis. An automated IPMS system is described in co-assigned U.S. patent application Ser. No. 10/094,394, entitled“A Method and Apparatus for Automated Analysis and Characterization of Chemical Constituents of Process Solutions,” filed Mar. 8, 2002, the contents of which are hereby incorporated by reference in their entirety. Although the IPMS technique provide accurate results, it may require the use of enriched isotopes of the species to be analyzed. Enriched isotopes are generally quite expensive, making continuous analysis expensive. Accordingly, there is another need in the art for improved mass spectrometry techniques that do not require the use of enriched isotope spikes.